This invention relates generally to photovoltaic cells, and more particularly the invention relates to the fabrication of such cells and the passivating antireflective coatings used thereon.
The back contact solar cell is a well-known device for converting solar energy to electrical energy. See Swanson et al., "Point Contact Silicon Solar Cells," IEEE Transactions on Electron Devices, Vol. ED-31, No. 5, May 1984, pages 661-664. The structure includes a lightly-doped or intrinsic silicon substrate in which alternating rows of p-doped regions and n-doped regions are formed in one major (back side) surface. Two metal layers respectively contact the p- and n-doped regions with the two metal layers being separated by an electrically insulating layer. Alternatively, interdigitated metal contacts provide electrical contacts to the doped regions and provide voltage output terminals for the cell in response to the generation of electron-hole pairs in the semiconductor material by impinging photons. The opposite major surface (front side) is covered by a passivation layer to minimize recombination of electrons and holes at the surface and provide high transmission of photons into the bulk material Conventionally, thermally-grown silicon oxide and deposited silicon nitride, or other transparent layers such as magnesium fluoride or zinc sulfide, have been used as passivation material.
A typical and normal method of producing a passivating, antireflection film or layer on photovoltaic or photoelectric material such as silicon utilizes thermal oxidation. The oxidation is usually produced by oxidizing the silicon prior to metallization at temperatures ranging from 850.degree. C. to 1200.degree. C., typically 1000.degree. C., in an oxygen atmosphere. The resulting oxide layer may be one-quarter wavelength of average light or, for example, 114 nanometers thick. Alternatively, a thinner layer of thermal oxide may be used upon which various layers of other antireflective material such as silicon nitride or magnesium fluoride are deposited to further maximize the right transmission.
The semiconductor substrate for the photovoltaic cell must be thin. Conventionally, a semiconductor wafer is initially thinned by etching prior to the fabrication of the doped regions and the metallization. A limiting factor in the thinning of the wafer is the requisite structural strength required of the wafer during the subsequent processing. Accordingly, the thickness of the wafer must be several hundred micrometers to ensure adequate structural strength.